Allow SD card sharing on SKR Mini

Marlin/src/pins/pins_BIGTREE_SKR_MINI_V1_1.h

   #define MOSI_PIN      PB5
   #define SS_PIN        PA15
 #else
+  #define SDCARD_CONNECTION ONBOARD
   #define ENABLE_SPI1
   #define SD_DETECT_PIN PA3
   #define SCK_PIN       PA5

Marlin/src/pins/pins_MKS_SBASE.h

 #define ENET_TXD0          P1_00   // J12-11
 #define ENET_TXD1          P1_01   // J12-12
 
-#ifndef SDARD_CONNECTION
+#ifndef SDCARD_CONNECTION
   #define SDCARD_CONNECTION ONBOARD
 #endif